PRNP and prion disease: Lymphatic tissue is the accepted tissue type for live animal testing of scrapie, a prion disease in sheep,1, 2 for which genetic variation within the prion gene (PRNP) confers resistance.3 Circulating lymphocytes infected with PrPSc can cause scrapie in sheep.4, 5 In cattle, a lymphocyte profile has been found to be associated with a PRNP genotype6 and, if true in sheep, may have implications for live animal testing and disease progression.